Detection of trace amounts of illicit substances such as explosives and narcotics is an ever more critical element of combating terrorism and contraband. However, the exacting operating requirements a detector must meet in order to be useful for these purposes severely limit the number of suitable technologies.
The primary operating requirement is sensitivity. Many of the materials targeted by law enforcement or security screenings are present in the gas phase at very low fractional molecular concentrations. Table 1 shows approximate values of room-temperature vapor pressures for common explosives and cocaine, reports of which often vary by as much as an order of magnitude.
Realistic use in demanding security environments, entailing screening thousands of containers per day, would further require that each determination be completed rapidly, in well under a second. For many law enforcement situations, a serviceable illicit substance detector would necessarily be portable.
Many sensing devices have been proposed for detecting trace amounts of explosives or drugs in security or law enforcement contexts. However, none has combined the sensitivity to detect constituents present at concentrations as low as 10xe2x88x9212 atm with the requisite rapidity and portability.
For example, a bioluminescence-based explosives detection and identification system capable of detecting constituents in air having a fractional molecular concentration on the order of 10xe2x88x9214 has been proposed. However, the required processing time is on the order of several minutes. (See, e g, E. M. Boncyk in Proc. 3rd Int. Symp. on Analysis and Detection of Explosives, Mannheim-Neuostheim, Germany, 4.1-40.14 [1989].)
Ion mobility spectrometry (xe2x80x9cIMSxe2x80x9d) has found wide application as a relatively quick and accurate technology for detecting explosives and illicit drugs. The nominal sensitivity of IMS-based systems ranges from about 10xe2x88x9212 to 10xe2x88x9214 atm. As is typical of gas-phase sensors, the minimum level of a target compound detectable by this technique is limited by false positives and interference from other gaseous constituents rather than by the inherent capability of the sensor.
The reliance of standard IMS on the ion charge-to-mass ratio to differentiate constituents predisposes it to false positives when used to detect explosives or drugs. For example, the ion mobility spectrum obtained from methamphetamine, a product of cocaine decomposition, overlaps on the time axis with that due to a common skin conditioner ingredient, so that this ingredient provokes a false positive by an ion mobility spectrometer configured to detect methamphetamine. Introducing an ionizable vapor dopant that neutralizes the problematic skin conditioner ingredient but not molecules of explosives or methamphetaminexe2x80x94which have exceptionally large electron or proton affinitiesxe2x80x94mitigates this difficulty, but at the expense of some increase in system complexity. Impurities remaining in the sensor from a previous screening are another significant source of error in IMS-based systems.
Also, substances such as RDX and PETN having vapor pressures near the lower limits of detectability by IMS can be detected by this method only after several seconds of sampling. Such an interval is unacceptably long for high-volume applications, such as comprehensive passenger screening at airports.
It is, accordingly, an object of the present invention to provide method and apparatus for enhancing the capability of detectors with respect to trace constituents.
It is another object of the present invention to provide method and apparatus or eliminating interfering background impurities prior to subsequent downstream detection.
It is another object of the invention to provide method and apparatus for rapidly detecting trace constituents.
It is another object of the invention to reduce the occurrence of false positives in ion mobility spectrometry systems.
It is another object of the invention to provide method and apparatus that allow easy and quick clearing of a sensor system.
It is yet another object of the invention to provide suitable method and apparatus for detecting illicit drugs and explosives and decomposition products thereof in law enforcement and security environments.
The invention provides method and apparatus for propelling a target chemical constituent, or equivalently a set of constituents, along a pathway by applying a time-varying temperature profile along the pathway so as to effect a dynamic Soret effect. The temperature profile impressed upon the pathway creates at least one region over which temperature varies with position, so as to produce a warmer zone and a cooler zone situated consecutively along the path. In accordance with the Soret effect, components present at dilute concentration in a carrier medium segregate in the temperature gradient according to their respective molecular weights. Components having molecular weights greater than that of the carrier medium accumulate in the cooler zone, whereas components having higher molecular weights diffuse toward the warmer zone. In moving to establish this thermally driven concentration gradient, each component advances toward the appropriate portion of the temperature profile at a respective net average velocity known to those of skill in the art as its Soret velocity.
In accordance with the invention, the region of temperature variation is displaced along the pathway at a wave velocity, so as to generate a time-varying temperature profile. As the local temperature changes, the segregated dilute components move so as to preserve or reestablish the thermally induced concentration gradient. Thus the components are conveyed along the pathway with the moving region of temperature variation. The quantity of a particular constituent that is pumped down the path depends on the temperature gradient, the absolute value of the wave velocity and its relative value compared to the constituent""s Soret velocity, and also the diffusion coefficient of the constituent in the carrier medium.
In one embodiment, the invention provides a dynamic thermophoretic concentrator for separating a target chemical constituent from a mixture of components on the basis of diffusion coefficient by using alternate forward and backward motion of the temperature profile along the pathway, thereby accumulating an ultimate concentration of the target constituent greater than its initial concentration in the mixture by a factor up to ten, 100, 103, 104 or even greater. Because most components have very similar Soret velocities, as a practical matter the distribution of a constituent across a given moving temperature profile depends mainly on its diffusion coefficient. Particles having small diffusion coefficients, correlating with large particle sizes, are concentrated in the cooler portion of the temperature profile more compactly, and thus transported at a greater flux by the time-varying profile; the degree of localization drops rather abruptly with increasing diffusion coefficient, so that smaller constituents are distributed more evenly throughout the region of varying temperature and less efficiently transported. The diffusion coefficient at which the flux declines can be shifted to higher values by increasing the temperature difference between the warmer and cooler extremes. For a given temperature gradient, the basic shape of the flux-diffusivity function changes with the wave velocity.
In accordance with the invention, the temperature profiles and wave velocities used for forward and backward motion are chosen in conjunction to enhance the net forward flux of a target constituent, and suppress that of other constituents, based on diffusivities, thereby preferentially conveying the target constituent forward. For example, distinct forward and backward thermal profiles, differing from one another in shape or temperature gradient, may be used. Or, in a preferred embodiment, a single temperature profile is moved at different forward and backward wave velocities. In particular, a target constituent is concentrated at an end of the pathway by alternately moving the temperature profile toward the end at a forward wave velocity greater than the constituent""s Soret velocity and away from the end at a backward wave velocity less than the constituent""s Soret velocity.
The invention is compatible with micrometer-scale implementation, allowing for a reduction in both concentration time and power requirement compared to concentrators known in the art. Its speed and portability suit it for security and law enforcement applications and its sensitivity is equal to detecting residues of explosives and narcotics or their decomposition products. To this end, the thermophoretic concentrator of the invention is particularly advantageous when used in conjunction with a fluid phase detector, for example an ion-mobility spectrometer. The use of diffusivity as a discriminator enables ion-mass spectrographic systems to differentiate between a target constituent and other, confounding dilute components, thereby abating false positive indications.
The ability to selectively preconcentrate an arbitrary dilute constituent by several orders of magnitude and also to eliminate background impurities before subsequent downstream detection mitigates the limitation on minimum detectable constituent level inherent to most gas-phase sensors. Furthermore, by wholesale backward pumping the dynamic thermophoretic concentrator of the invention is able to clear the system of extraneous impurities between uses in less time than is required to detect a target constituent.